Interaction of water with a fatty acid Langmuir film

Using sum-frequency vibrational spectroscopy we have studied the structure of the fatty acid monolayer–water interface as a function of pH of water. At low pH values, the monolayer is neutral and the surface water structure is disordered by hydrogen bonding of water molecules with the acid head-groups. At high pH values, acid head-groups are ionized and the resulting surface field helps the surface water molecules form a more ordered hydrogen-bonding network. At pH ∼3, a small change in pH leads to a reorientation of the acid OH groups while the alkyl chains remain conformationally ordered at all pH values.